Considerations When Using Plastic Gears

Engineers and designers can’t view plastic material gears as just steel gears cast in thermoplastic. They must pay attention to special issues and factors unique to plastic gears. In fact, plastic gear style requires attention to details which have no effect on metallic gears, such as for example heat build-up from hysteresis.

The essential difference in design philosophy between metal and plastic gears is that metal gear design is founded on the strength of an individual tooth, while plastic-gear design recognizes load sharing between teeth. Put simply, plastic teeth deflect even more under load and pass on the load over more teeth. Generally in most applications, load-sharing increases the load-bearing capacity of plastic gears. And, consequently, the allowable tension for a specified number-of-cycles-to-Screw Vacuum Pumps failure boosts as tooth size deceased to a pitch of about 48. Little increase is seen above a 48 pitch because of size effects and various other issues.

In general, the next step-by-step procedure will create a good thermoplastic gear:

Determine the application’s boundary conditions, such as temperature, load, velocity, space, and environment.
Examine the short-term material properties to determine if the original performance levels are adequate for the application.
Review the plastic’s long-term property retention in the specified environment to determine if the performance amounts will be preserved for the life of the part.
Calculate the stress amounts caused by the many loads and speeds using the physical property or home data.
Compare the calculated values with allowable worry amounts, then redesign if had a need to provide an sufficient safety factor.
Plastic gears fail for most of the same reasons metal types do, including wear, scoring, plastic flow, pitting, fracture, and fatigue. The cause of these failures is also essentially the same.

One’s teeth of a loaded rotating gear are subject to stresses at the root of the tooth and at the contact surface area. If the gear is usually lubricated, the bending stress is the most important parameter. Non-lubricated gears, on the other hand, may degrade before a tooth fails. Therefore, contact stress may be the prime factor in the design of these gears. Plastic gears will often have a complete fillet radius at the tooth root. Therefore, they aren’t as susceptible to stress concentrations as steel gears.

Bending-stress data for engineering thermoplastics is based on fatigue tests work at specific pitch-series velocities. Therefore, a velocity factor should be found in the pitch line when velocity exceeds the check speed. Constant lubrication can boost the allowable tension by one factor of at least 1.5. As with bending tension the calculation of surface area contact stress takes a number of correction elements.

For instance, a velocity element can be used when the pitch-line velocity exceeds the test velocity. Furthermore, a factor can be used to account for changes in operating temperature, gear materials, and pressure angle. Stall torque is usually another factor in the design of thermoplastic gears. Often gears are at the mercy of a stall torque that is substantially higher than the standard loading torque. If plastic material gears are run at high speeds, they become vulnerable to hysteresis heating which may get so severe that the gears melt.

There are several methods to reducing this kind of heating. The favored way is to reduce the peak stress by increasing tooth-root area available for the mandatory torque transmission. Another approach is to reduce stress in the teeth by increasing the apparatus diameter.

Using stiffer materials, a material that exhibits less hysteresis, can also prolong the operational existence of plastic-type material gears. To improve a plastic’s stiffness, the crystallinity levels of crystalline plastics such as for example acetal and nylon can be increased by processing techniques that increase the plastic’s stiffness by 25 to 50%.

The most effective approach to improving stiffness is by using fillers, especially glass fiber. Adding glass fibers increases stiffness by 500% to 1 1,000%. Using fillers has a drawback, though. Unfilled plastics have fatigue endurances an purchase of magnitude greater than those of metals; adding fillers decreases this benefit. So engineers who wish to make use of fillers should take into account the trade-off between fatigue existence and minimal high temperature buildup.

Fillers, however, perform provide another benefit in the ability of plastic gears to resist hysteresis failing. Fillers can increase temperature conductivity. This helps remove high temperature from the peak tension region at the base of the gear tooth and helps dissipate warmth. Heat removal may be the additional controllable general element that can improve level of resistance to hysteresis failure.

The surrounding medium, whether air or liquid, has a substantial effect on cooling prices in plastic gears. If a fluid such as an essential oil bath surrounds a equipment instead of air, temperature transfer from the gear to the natural oils is usually 10 occasions that of the heat transfer from a plastic material gear to air flow. Agitating the essential oil or air also increases heat transfer by a factor of 10. If the cooling medium-again, atmosphere or oil-is certainly cooled by a temperature exchanger or through design, heat transfer increases a lot more.

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